Over the past few decades, camera optics have become increasingly complex. For example, the lenses of modern single lens reflex (SLR) cameras may contain a dozen or more individual lens elements, which are used to optimize light efficiency of the optical system while minimizing aberrations, i.e., non-linear deviations from an idealized thin lens model.
Optical aberrations include effects such as geometric distortions, chromatic aberration (wavelength-dependent focal length), spherical aberration (where focal length depends on the distance from the optical axis), and coma (angular dependence on focus). Single lens elements with spherical surfaces typically suffer from these artifacts, and as a result may not be used in high-resolution, high-quality photography. Instead, modern optical systems often feature a combination of different lens elements with the intent of canceling out aberrations. For example, an achromatic doublet is a compound lens made from two glass types of different dispersion, i.e., their refractive indices depend on the wavelength of light differently. The result is a lens that is (in the first order) compensated for chromatic aberration, while still suffering from the other artifacts mentioned above.
Despite typically having better geometric imaging properties, modern lens designs are often not without disadvantages, including a significant impact on the cost and weight of camera objectives, as well as increased lens flare.
That is, modern imaging optics can be highly complex systems with up to two dozen individual optical elements. This complexity is normally required in order to compensate for the geometric and chromatic aberrations of a single lens, including geometric distortion, field curvature, wavelength-dependent blur, and color fringing.